1. Field of the Invention
The present invention relates to apparatus for forming a color image (on a recording medium or for viewing) from an electrical signal and more particularly to improvements in electronic color imaging apparatus of the kind using light valve arrays.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,229,095 discloses various embodiments of electronic color-imaging apparatus that utilize arrays of separately-addressable, pixel (picture element) sized, electro-optical means to effect multicolor exposure of panchromatic imaging media. One preferred kind of electro-optical means disclosed in that patent is a light valve comprising a panel of ferro-electric ceramic material, such as lanthanum doped lead zirconate titanate (PLZT) sandwiched between crossed polarizers and activated to operate in a quadratic Kerr cell mode. Thus an array of such light valves comprises a panel of PLZT material with a plurality of interleaved electrodes formed on one major surface in a manner facilitating the selective application of discrete electrical fields transversely across (in a direction perpendicular to the direction of viewing) discrete surface areas of the plate. Upon application of such fields, the PLZT material becomes birefringent and rotates the direction of polarization of incident light by an extent dependent on the field magnitude. This results in the transmission of light through the PLZT panel and polarizers varying as a function of the electric fields. A color image is formed electronically by selectively opening and closing of such light valves in synchronization with pulses of red, green and blue light and according to the red, green and blue color information for the pixels of that image.
The above-mentioned patent discloses two preferred approaches for providing such successive pulses of different color light. In one approach separate radiation sources, such as separate elongated flash lamps with respective red, green and blue filters, are energized successively by a controllable power source. In the other approach a rotating member, having successive red, green and blue filter sectors is interposed between a single illumination source and the light valve array.
The above approaches are completely suitable for many electronic imaging applications; however, in certain electronic imaging applications they each have some undesirable features. The multiple-lamp approach involves three, instead of one, illuminating sources and thus some additional costs. Moreover, there are certain limits as to how fast these sources can be turned on and off and for higher speed imaging applications, this can present a problem. The rotating filter approach avoids both of the aforementioned potential difficulties; however, it involves precise synchronization of the filters' movement and thus involves the cost of precise servo-systems, as well as potential operational difficulties in higher speed applications.
Alternative approaches for providing multicolor exposures in accordance with the teachings of that patent are to provide three separate imaging stations, spaced along the path of movement of the photosensitive medium or to provide for at least three imaging passes of the photosensitive medium relative to a single imaging station.
In the multiple-spaced-station approach, each different station is provided with a linear array uniformly illuminated with a different color of light (e.g., red, green or blue) and the different arrays are each addressed to an on-off condition according to the line-color pertaining to the line of the photosensitive medium currently aligned therewith. For example, if the blue color array is furthest downstream it will be addressed with line-one information while the furthest upstream red color array is addressed with line-three information, having already exposed lines one and two with red light. Although useful, this approach has disadvantages because it is difficult to place and maintain the separate arrays in proper relative alignment with respect to the imaging path. Thus, the light valve pixels are minute in size, e.g. 200/linear inch, and without great care, the pixels will be in improper transverse alignment (e.g., pixels 3, 4, 5 of the red array aligned with pixels 4, 5, 6 of the green array and 2, 3, 4 of the blue array). It will be appreciated that when such misaligned arrays are addressed by the control electronics a given pixel, e.g. 4 of the image medium, could receive the red exposure for pixel 4 but the green exposure for pixel 5 and the blue exposure for pixel 3. The same problem exists if separate arrays are used for multiple-pass color separation exposures and if a single array is used it is necessary to change color filters or light sources.